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COLOR MODELS 1

CONTENTS

The Two Most Common Color Models .................................................................... 1

Additive vs. Subtractive Color Models ................................................................... 2

RGB Color Model ......................................................................................... 2

CMYK Color Model ........................................................................................ 3

Color Gamut and Color "Space" ........................................................................... 3

RGB Color .................................................................................................... 4

CMYK or "Process Color" ................................................................................... 4

When In Doubt, Save Images As RGB .................................................................... 5

Use RGB For Screen Displays and CMYK For Print ................................................... 5

Summary ..................................................................................................... 6

A color model is an orderly system for creating a whole range of colors from a small set of

primary colors. There are two types of color models, those that are subtractive and those

that are additive. Additive color models use light to display color while subtractive models

use printing inks. Colors perceived in additive models are the result of transmitted light.

Colors perceived in subtractive models are the result of reflected light.

THE TWO MOST COMMON COLOR MODELS

There are several established color models used in computer graphics, but the two most

common are the RGB model (Red-Green-Blue) for computer display and the CMYK model

(Cyan-Magenta-Yellow-blacK) for printing.

1 http://www.sketchpad.net/basics4.htm

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Notice the centers of the two color charts. In the RGB model, the convergence of the three

primary additive colors produces white. In the CMYK model, the convergence of the three

primary subtractive colors produces black.

In the RGB model notice that the overlapping of additive colors (red, green and blue) results

in subtractive colors (cyan, magenta and yellow). In the CMYK model notice that the

overlapping of subtractive colors (cyan, magenta and yellow) results in additive colors (red,

green and blue).

Also notice that the colors in the RGB model are much brighter than the colors in the CMYK

model. It is possible to attain a much larger percentage of the visible spectrum with the RGB

model. That is because the RGB model uses transmitted light while the CMYK model uses

reflected light. The muted appearance of the CMYK model demonstrates the limitation of

printing inks and the nature of reflected light. The colors in this chart appear muted because

they are displayed within their printable gamut (see below).

ADDITIVE VS. SUBTRACTIVE COLOR MODELS

Since additive color models display color as a result of light being transmitted (added) the

total absence of light would be perceived as black. Subtractive color models display color as a

result of light being absorbed (subtracted) by the printing inks. As more ink is added, less and

less light is reflected. Where there is a total absence of ink the resulting light being reflected

(from a white surface) would be perceived as white.

RGB COLOR MODEL

RGB Color Model

Additive color model

For computer displays

Uses light to display color

Colors result from transmitted light

Red+Green+Blue=White

CMYK Color Model

Subtractive color model

For printed material

Uses ink to display color

Colors result from reflected light

Cyan+Magenta+Yellow=Black

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Additive color model

For computer displays

Uses light to display color

Colors result from transmitted light

Red+Green+Blue=White

CMYK COLOR MODEL

Subtractive color model

For printed material

Uses ink to display color

Colors result from reflected light

Cyan+Magenta+Yellow=Black

COLOR GAMUT AND COLOR "SPACE"

Each color model has is own gamut (range) of colors that can be displayed or printed. Each

color model is limited to only a portion of the visible spectrum. Since a color model has a

particular range of available color or gamut, it is referred to as using a "color space". An

image or vector graphic is said to use either the RGB color space or the CMYK color space (or

the color space of another color model). Some graphic applications present the user with

more than one color model for image editing or illustration and it is important to choose the

right one for the task. The whole point of this article is to explain the difference between the

two color models so you choose the right one for the job. For you work to display at its best,

choosing the right color model is critical.

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RGB COLOR

The RGB model forms its gamut from the primary additive colors of red, green

and blue. When red, green and blue light is combined it forms white.

Computers generally display RGB using 24-bit color. In the 24-bit

RGB color model there are 256 variations for each of the

additive colors of red, green and blue. Therefore there are

16,777,216 possible colors (256 reds x 256 greens x 256

blues) in the 24-bit RGB color model.

In the RGB color model, colors are represented by

varying intensities of red, green and blue light. The

intensity of each of the red, green and blue

components are represented on a scale from 0 to 255

with 0 being the least intensity (no light emitted) to 255

(maximum intensity). For example in the above RGB chart the magenta color would be R=255

G=0 B=255. Black would be R=0 G=0 B=0 (a total absence of light).

CMYK OR "PROCESS COLOR"

The CMYK printing method is also known as "four-color process" or simply "process" color. All

of the colors in the printable portion of the color spectrum can be achieved by overlapping

"tints" of cyan, magenta, yellow and black inks. A tint is a screen of tiny dots appearing as a

percentage of a solid color. When various tints of the four colors are printed in overlapping

patterns it gives the illusion of continuous tones - like a photograph:

The CMYK model forms its gamut from the primary subtractive

colors of cyan, magenta and yellow. When cyan, magenta and

yellow inks are combined it forms black - in theory.

However, because of the impurities in ink, when cyan,

magenta and yellow inks are combined it produces

a muddy brown color. Black ink is added to this system

to compensate for these impurities. In the

CMYK color model, colors are represented as

percentages of cyan, magenta, yellow and black. For

example in the above CMYK chart the red color is

composed of 14% cyan, 100% magenta, 99% yellow and 3% black.

White would be 0% cyan, 0% magenta, 0% yellow and 0% black (a total absence of ink on white

paper).

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WHEN IN DOUBT, SAVE IMAGES AS RGB

The RGB model displays a much larger percentage of the visible spectrum than the CMYK

model and, as a result, has a wider gamut. Once an image has been converted from RGB to

CMYK and brought into printable gamut, the extra RGB data will be lost.

One can retain out-of-gamut areas in CMYK images and leave it to the computer to bring the

colors into gamut at printing time. However this requires the computer to make the

conversion from RGB to CMYK and this doesn't always work out as well as one might expect.

Conversion between color models is not always a good idea.

Because of this fact, you want to scan or shoot images (with a digital camera) using the

appropriate color model for their primary purpose. If the images will be used primarily for

print then use CMYK. If they will be used primarily for screen displays, then use RGB. You can

always convert from RGB to CMYK (or vise-versa) but it is best not to.

If you can afford the time, money and disk space to scan or shoot both versions of an image

where both are needed, then this is the best solution. This is especially true if you will be

using the same images for both printed material (such as a catalog) and the web (such as an

online catalog). A little planning can go a long way here.

But if you are not sure, then I generally recommend saving images in RGB mode and creating

CMYK copies for printed material as needed.

USE RGB FOR SCREEN DISPLAYS AND CMYK FOR PRINT

It is important to choose the right color model for the job. If your images will be printed,

then convert them to CMYK and manually bring them into gamut before printing. If your

images are to be displayed on a computer, then make sure you use RGB color so the full

gamut will be available for display. Because both models can be available at the same time

while using an application, it is easy to make a mistake and choose the wrong palette or set

of color swatches.

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SUMMARY

• RGB

o Additive

• CMYK

o Subtractive